Multilayered Coating for Downhole Tools with Enhanced Wear Resistance and Acidic Corrosion Resistance

ABSTRACT

A coating for protecting a base material from wear and corrosion includes a first layer deposited directly onto an outer surface of the base material. In addition, the coating includes a second layer deposited directly onto the first layer. The first layer is positioned between the base material and the second layer. The first layer includes chromium having a first micro-crack density and the second layer comprises chromium having a second micro-crack density that is less than the first micro-crack density.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Application Ser. No. 61/904,287 filed Nov. 14, 2013, and entitled “Multilayered Coating for Downhole Tools with Enhanced Wear Resistance and Acidic Corrosion Resistance,” which is hereby incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND

The present disclosure relates generally to a multi-layered coating for downhole tools and earth-boring drill bits used to drill a borehole for the ultimate recovery of oil, gas, or minerals. More particularly, the present disclosure relates to a multilayered protective coating that provides enhanced wear resistance and acid corrosion resistance for downhole tools, such as, but not limited to mandrels, mud motor rotors and agitator rotors, and drill bits.

An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by rotating the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole thus created will have a diameter generally equal to the diameter or “gage” of the drill bit.

The cost of drilling a borehole for recovery of hydrocarbons is very high, and is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is affected by the number of times the drill bit must be changed before reaching the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipe, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. This process, known as a “trip” of the drill string, requires considerable time, effort and expense. Accordingly, it is desirable to employ drill bits that will drill faster and longer. The length of time that a drill bit may be employed before it must be changed depends upon a variety of factors, including the bit's rate of penetration (“ROP”), as well as its durability or ability to maintain a high or acceptable ROP. In turn, ROP and durability are dependent upon a number of factors, including the ability of the bit body to resist abrasion, erosion, and impact loads.

Two predominant types of drill bits are roller cone bits and fixed cutter bits, also known as rotary drag bits. Bit performance is often limited by selective/localized wear and corrosive damage to the bit body. Excessive wear and corrosive damage can alter and negatively affect specific design parameters for optimal cutting and hydraulic flow paths. For example, excessive localized wear can alter cutter exposure (i.e., extension height of cutter elements). As another example, excessive wear and corrosion around cutter elements can increase the likelihood of such cutter elements being broken off or otherwise removed from the bit during drilling operations. In addition, oil and gas wells are often severely corrosive environments. In particular, oil and natural gas contain corrosive substances such as carbon dioxide gas, hydrogen sulfide, and chlorine ions. Prolonged exposure to such corrosive substances can weaken and damage drill bits, thereby reducing their durability and useful life.

To improve the wear and corrosive resistance of bit bodies, a protective coating can be applied to the base metal (steel) of the bit body. Hard chromium plating has been used as a protective coating for downhole tools in oil and gas industry. Conventional hard chrome plating is applied in areas of need via electroplating and typically has a thickness of about 0.005 in. As is known in the art, electroplating refers to the electrolytic deposition of a layer of metal onto a base metal. Electroplating is performed in a plating bath containing a liquid solution (or electrolyte) including the desired plating metal dissolved as microscopic particles (positive charged ions) suspended in a conductive solution. The object to be plated is submerged in the plating bath and a low voltage DC current is applied to the bath. Generally located at the center of the plating bath, the object to be plated acts as a negatively charged cathode. The positively charged anodes complete the DC circuit. A power source known as a rectifier is used to convert AC power to a carefully regulated low voltage DC current. The resulting circuit channels the electrons into a path from the rectifier to the cathode (surface being plated), through the plating bath to the anode (positively charged) and back to the rectifier. The positively charged ions at the anodes flow through the plating bath's metal electrolyte toward the negatively charged cathode. This movement causes the metal ions in the bath to migrate toward extra electrons located at the cathode's surface outer layer. By means of electrolysis, the metal ions are taken out of solution and are deposited as a layer onto the surface of the surface of a downhole tool. This process is often termed “electrodeposition,” and the thickness of the electroplated layer deposited on the surface is determined by the time of plating, the amount of available metal ions, and the current density (A/in²) applied.

BRIEF SUMMARY OF THE DISCLOSED EMBODIMENTS

In one embodiment disclosed herein, a coating for protecting a base material from wear and corrosion comprises a first layer deposited directly onto an outer surface of the base material. In addition, the coating comprises a second layer deposited directly onto the first layer. The first layer is positioned between the base material and the second layer. The first layer comprises chromium having a first micro-crack density and the second layer comprises chromium having a second micro-crack density that is less than the first micro-crack density. In one embodiment of the coating, the first micro-crack density is greater than 1000 micro-cracks per inch and the second micro-crack density is between 400 and 650 micro-cracks per inch.

In another embodiment of the coating, the first layer has a first thickness measured perpendicular to the outer surface of the base material and the second layer has a second thickness measured perpendicular to the outer surface of the base material; where the first thickness of the first layer is substantially uniform and the second thickness of the second layer is substantially uniform. In a further embodiment of the coating the second layer has a thickness less than 0.0050 in. measured perpendicular to the outer surface of the base material, and in a still further embodiment of the coating the second layer has a thickness between about 0.00050 in. and about 0.0020 in. measured perpendicular to the outer surface of the base material.

In yet another embodiment of the coating, the first layer has a first hardness greater than 1000 HV and the second layer has a second hardness of about 850 HV. In another embodiment, the coating has an inner surface engaging the outer surface of the base material and an outer surface distal to the base material, and wherein the coating has a total thickness less than 0.030 in. measured perpendicularly from the outer surface of the base material to the outer surface of the coating; and in a further embodiment the first layer has a thickness between about 0.00020 in. and 0.0030 in. measured perpendicular to the outer surface of the base material.

In one embodiment disclosed herein, a method for forming a wear and corrosion resistant coating on a surface of a base material comprises (a) depositing a first layer of a first material onto the surface of the base material. In addition, the method comprises (b) depositing a second layer of a second material onto the first layer after (a). The first material or the second material comprises chromium and is deposited at a current density of less than about 4.0 A/in2. In a further embodiment of the method, the first material and the second material each comprise chromium, wherein (a) comprises depositing the first layer of the first material at a first current density; and wherein (b) comprises depositing the second layer of the second material at a second current density that is different than the first current density. In a further still embodiment of the method, one of the second current density and the first current density is about 3.5 A/in2 and the other of the first current density and the second current density is about 1.0 A/in2. In one embodiment of the method (a) comprises depositing the first layer of the first material by pulse current; and (b) comprises depositing the second layer of the second material by pulse current; in another embodiment (b) comprises depositing the second layer of the second material until the second layer has a thickness of about 0.0002 in. to 0.0030 in. measured perpendicular to the surface of the base material; in a further embodiment (a) comprises depositing the first layer of the first material until the first layer has a thickness of about 0.00020 in. to 0.0030 in. measured perpendicular to the surface of the base material, and in a further still embodiment the first material consists of chromium and the second material consists of chromium. In one embodiment of the method, the first material comprises Ni—P and the second material comprises chromium, and in another embodiment of the method (a) comprises depositing the first layer of the first material by an electroless process; in a further embodiment of the method (b) comprises depositing the second layer of the second material until the second layer has a thickness of about 0.0002 in. to 0.0030 in. measured perpendicular to the surface of the base material; and in a further still embodiment of the method comprises (c) heating the first layer after (a) and before (b) wherein (c) comprises: (c1) heating the first layer at about 375° F. for about 1.5 hr.; and (c2) heating the first layer at about 500° F. for about 1 hr after (c1). In another embodiment of the method (c) comprises increasing the hardness of the first layer to about 50 Rc.

In yet another embodiment disclosed herein, a down-hole tool comprises a body made of a base material. In addition, the down-hole tool comprises a protective coating mounted to an outer surface of the base material. The protective coating comprises a first layer deposited directly onto an outer surface of the base material and a second layer deposited directly onto the first layer. The first layer is positioned between the base material and the second layer. The first layer comprises chromium having a first micro-crack density and the second layer comprises chromium having a second micro-crack density that is less than the first micro-crack density. In one embodiment of the tool, the first micro-crack density is greater than 1000 micro-cracks per inch and the second micro-crack density is between 400 and 650 micro-cracks per inch; in another embodiment of the tool, the first layer has a first thickness measured perpendicular to the outer surface of the base material and the second layer has a second thickness measured perpendicular to the outer surface of the base material; wherein the first thickness of the first layer is substantially uniform and the second thickness of the second layer is substantially uniform; in a further embodiment, the second layer has a thickness between about 0.00050 in. and about 0.0020 in. measured perpendicular to the outer surface of the base material; and in a further still embodiment the coating has an inner surface engaging the outer surface of the base material and an outer surface distal the base material, and wherein the coating has a total thickness less than 0.030 in. measured perpendicularly from the outer surface of the base material to the outer surface of the coating. In another embodiment of a tool provided for herein, the first layer has a first hardness greater than 1000 HV and the second layer has a second hardness of about 850 HV.30.

In a further embodiment disclosed herein, a coating for protecting a base material from wear and corrosion comprises a first layer deposited directly onto an outer surface of the base material. In addition, the coating comprises a second layer deposited directly onto the first layer. The first layer is positioned between the base material and the second layer. The first layer comprises Ni—P and the second layer comprises chromium having a micro-crack density between 400 and 650 micro-cracks per inch. In another embodiment the second layer consists of chromium; in a further embodiment the first layer has a first thickness measured perpendicular to the outer surface of the base material and the second layer has a second thickness measured perpendicular to the outer surface of the base material; wherein the first thickness of the first layer is substantially uniform and the second thickness of the second layer is substantially uniform; and in a further still embodiment the first layer has a first thickness measured perpendicular to the outer surface of the base material and the second layer has a second thickness measured perpendicular to the outer surface of the base material; wherein the first thickness is between 0.0005 in. and 0.002 in. and the second thickness is between 0.0002 in. and 0.0030 in. In another embodiment, the coating has an inner surface engaging the outer surface of the base material and an outer surface distal the base material, and wherein the coating has a total thickness less than 0.030 in. measured perpendicularly from the outer surface of the base material to the outer surface of the coating.

Embodiments described herein comprise a combination of features and advantages intended to address various shortcomings associated with certain prior devices, systems, and methods. The foregoing has outlined rather broadly the features and technical advantages of the invention in order that the detailed description of the invention that follows may be better understood. The various characteristics described above, as well as other features, will be readily apparent to those skilled in the art upon reading the following detailed description, and by referring to the accompanying drawings. It should be appreciated by those skilled in the art that the conception and the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the disclosed embodiments of the invention, reference will now be made to the accompanying drawings, wherein:

FIG. 1 is a perspective view of an embodiment of a fixed cutter drill bit made in accordance with principles described herein;

FIG. 2 is a schematic cross-sectional view of the multilayered protective coating of FIG. 1;

FIG. 3 is a schematic cross-sectional view of the multilayered coating of FIG. 1, made in accordance with principles described herein;

FIG. 4A is an SEM image of the surface morphology a conventional hard chromium layer of the prior art comprising a base material 500, and a hard chrome layer 501;

FIG. 4B is an SEM image of the surface morphology of a multilayered chrome coating made in accordance with principles described herein, and comprising a base material 400, a first chrome layer 401, and a second chrome layer 402;

FIG. 5 is a process flow chart illustrating an embodiment of a method for making a multilayered chromium coating in accordance with principles described herein; and

FIG. 6 is a process flow chart illustrating an embodiment of a method for making a protective coating comprising both Ni—P and chromium layers in accordance with principles described herein.

DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENTS

The following discussion is directed to various exemplary embodiments of the invention. However, the embodiments disclosed should not be interpreted, or otherwise used, as limiting the scope of the disclosure, including the claims. In addition, one skilled in the art will understand that the following description has broad application, and the discussion of any embodiment is meant only to be exemplary of that embodiment, and that the scope of this disclosure, including the claims, is not limited to that embodiment.

The drawing figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may be omitted in interest of clarity and conciseness.

Certain terms are used throughout the following description and claims to refer to particular features or components. As one skilled in the art will appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name but not function. The drawing figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in interest of clarity and conciseness.

In the following discussion and in the claims, the terms “including” and “comprising” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to . . . . ” Also, the term “couple” or “couples” is intended to mean either an indirect or direct connection. Thus, if a first device couples to a second device, that connection may be through a direct connection, or through an indirect connection via other devices, components, and connections. In addition, as used herein, the terms “axial” and “axially” generally mean along or parallel to a central axis (e.g., central axis of a body or a port), while the terms “radial” and “radially” generally mean perpendicular to the central axis. For instance, an axial distance refers to a distance measured along or parallel to the central axis, and a radial distance means a distance measured perpendicular to the central axis. As used herein, the term “about,” when used in conjunction with a percentage or other numerical amount, means plus or minus 10% of that percentage or other numerical amount. For example, the term “about 80%,” would encompass 80% plus or minus 8%. The term Chromium, Cr and Chrome may be used interchangeably to describe some embodiments of the materials described herein. The terms “plating” and “coating” may be used interchangeably to describe embodiments of the materials described herein. The term “substantially” as used herein (unless specifically defined for a particular context elsewhere or the context clearly dictates otherwise) means nearly totally or completely, for instance, satisfying one or more of the following: greater than 50%, 51% or greater, 75% or greater, 80% or greater, 90% or greater, and 95% or greater of the condition.

As previously described, hard chromium plating has been used as a protective coating for downhole tools in oil and gas industry to improve the wear and corrosive resistance of bit bodies, a protective coating can be applied to the base metal (steel) of the bit body. While hard chrome plating enhances the wear and corrosion resistance, the conventional plating technique often introduces defects or cracks into the coating, for example when internal stress exceeds the tensile stress of the chromium. Further, in addition to the desired reaction resulting in the metallic chromium formation, many undesired side reactions occur. One of these is the formation of hydrogen gas, which can become entrapped and cause internal stresses, as well as subsequent cracking, as it seeks to escape the deposit. The width, depth, and population density of these cracks varies widely and is influenced by the following: the type of plating chemistry used (single-catalyst, mixed catalyst, proprietary), chromic acid concentration, type and concentration of catalyst, chromium-to-catalyst ratio, plating current-density, bath temperature, concentration of bath impurities (iron, copper, zinc, nickel, trivalent chromium, etc.), and the chromium deposit thickness surface condition of substrate.

Generally speaking, a micro-crack structure comprised of a high population density of narrow, shallow cracks is preferred because the deposit tends to have a lower stress, higher lubricity, good wearability and better corrosion resistance. If the conditions during plating cause the cracks to be coarse in nature, often referred to as macro-cracks, they may be visible to the naked eye. Usually, chromium with a microstructure comprising macro-cracks exhibits less desirable properties in service. For instance, corrosive fluids can more easily access the underlying substrate material through large macro-cracks than smaller micro-cracks.

A thin dense chromium (TDC) coating with thickness of 0.0001 in. or 0.0003 in. has less structural defects as compared to hard chrome plating, and is often void of micro-cracks. Thus, TDC usually exhibits better corrosion resistance than hard chrome plating. Thin dense chromium plating has been used in various coating applications such as bearing races, seal surfaces, pump piston, valves and pump housings, due primarily to its high surface hardness, low friction coefficient and high corrosion resistance. However, TDC is typically limited to a maximum thickness of about 0.0005 in., which results in a marked reduction in abrasion, erosion, abrasive wear, scuffing and galling. Therefore, TDC is less than ideal for highly abrasive and corrosive environments.

Embodiments disclosed herein provide coatings, compositions, and methods to protect and improve the wear and corrosive resistance downhole tools in oil and gas industry while offering the potential to overcome some of the foregoing challenges.

Referring now to FIG. 1, an embodiment of a downhole tool 100 in accordance with the principles described herein is shown. In this embodiment, tool 100 is a fixed cutter PDC bit adapted for drilling through formations of rock to form a borehole. Bit 100 has a central axis 105 about which it is rotated in a cutting direction 106 to drill the borehole. In addition, bit 100 includes a bit body 110, a shank 111, and an externally threaded connection or pin 112 attached to shank 111. Pin 112 connects bit 100 to a drill string (not shown). Bit body 110 has a bit face 120 formed on the end of the bit 100 that faces the formation and is generally opposite pin 112.

A cutting structure 121 is provided on face 120 and includes a plurality of circumferentially-spaced blades 130 that extend from bit face 120. In this embodiment, cutting structure 121 includes six angularly-spaced blades 130. Blades 130 are integrally formed as part of, and extend from, bit body 110 and bit face 120. Each blade 130 includes a cutter-supporting surface 131 for mounting a plurality of cutter elements 132. Each cutter element 132 comprises a cutting face 133 attached to an elongated and generally cylindrical support member or substrate 134, which is received and secured in a pocket formed in surface 131 of the corresponding blade 130 to which it is fixed. Each cutting face 133 is made of a very hard material, such as a polycrystalline diamond material, suitable for engaging and shearing the formation.

Bit 100 also includes circumferentially-spaced gage pads 140 disposed about the circumference of bit 100. In this embodiment, gage pads 140 are integrally formed as part of the bit body 110, with each gage pad 140 extending axially from a corresponding blade 130. Each gage pad 140 has a radially outer gage-facing surface 141 that slidingly engages the borehole sidewall during drilling to help maintain the size of the borehole and stabilize bit 100 against vibration. In certain embodiments, gage pads 140 include flush-mounted or protruding cutter elements embedded in gage-facing surfaces 141 to resist pad wear and assist in reaming the borehole sidewall.

To enhance the durability and operating lifetime of bit 100, select regions of bit body 110 are provided with a multilayered coating for protecting the base material (for example the metal forming bit body 110) from wear and corrosion, thereby providing enhanced wear resistance and corrosion resistance as described herein. Since formation facing surfaces 131 of blades 130 and gage-facing surfaces 141 of pads 140 are particularly susceptible to wear and damage, in this embodiment, a multilayered protective coating 150 that enhances resistance to wear and corrosion is provided on the entire formation facing surface 131 of each blade 130 and the entire gage-facing surface 141 of each gage pad 140. In other embodiments, additional surfaces of the bit body (e.g., bit body 110) can comprise multilayered protective coatings.

Referring now to FIG. 2, coating 150 is shown applied to the outer surface 154 of the base metal or material 153 of bit body 110. In general, coating 150 functions to protect underlying base material 153 from wear and corrosion during downhole operations. Coating 150 includes a plurality of layers, and thus, may also be referred to as “multilayered.” In particular, coating 150 includes a first layer 151 deposited directly onto the outer surface 154 of base material 153 and a second layer 152 deposited directly onto the first layer 151. Thus, first layer 151 is positioned between base material 153 and second layer 152.

First layer 151 comprises a first material 151 a and has a first thickness T₁₅₁ measured perpendicular to outer surface 154, and second layer 152 comprises a second material 152 a and has a second thickness T₁₅₂ measured perpendicular to outer surface 154. In this embodiment, first material 151 a and second material 152 a each comprise chromium. As will be described in more detail below, each chrome layer 151, 152 is applied via an electrolytic process at a different, discrete current density (e.g., A/in²).

In this embodiment, thickness T₁₅₁ of layer 151 is substantially constant and uniform moving laterally along coating 150, and thickness T₁₅₂ of layer 152 is substantially constant and uniform moving laterally along coating 150. The thickness of each layer 151, 152, respectively, is preferably less than 0.0050 in. More specifically, thickness T151 is preferably between about 0.00020 in. and 0.0030 in. and thickness T152 is preferably between about 0.00050 in. and about 0.0050 in. Each layer 151, 152 includes a plurality of micro-cracks. In general, the micro-cracks in a given layer 151, 152 can be oriented substantially parallel to the outer surface 154 of the base material or substantially perpendicular to the outer surface 154 of the base material 153. and/or oriented substantially perpendicular to the outer surface of the base material. The quantity or volume of micro-cracks in each layer 151, 152 can be characterized in terms of a “micro-crack density”, which refers to the average number of micro-cracks per unit length (e.g., micro-cracks per inch). A micro-crack is as known in the art, a crack in the material that is not visible to the naked eye, thus requiring a microscope (such as but not limited to SEM) to visualize the crack. A macro-crack in comparison is visible to naked eye (unaided human visual perception), and is thus greater than about 55 micrometers). In general, the micro-crack density of a layer or material can be measured or determined by microscope or such techniques familiar to one skilled in the art. The micro-crack density is inversely related to the current density at which the material is deposited, wherein a low current density will create a high micro-crack density, and high current density will produce a low micro-crack density.

In this embodiment, first layer 151 has a first micro-crack density, and second layer 152 has a second micro-crack density that is less than the first micro-crack density. In other words, second layer 152 has more micro-cracks per unit length than first layer 151. More specifically, in this embodiment, the first micro-crack density (of layer 151) is greater than 1000 micro-cracks per inch and the second micro-crack density (of layer 152) is between 400 and 650 micro-cracks per inch.

Referring still to FIG. 2, coating 150 has an inner surface 156 engaging outer surface 154 of base material 153, an outer surface 158 distal to the base material 153, and a total thickness T₁₅₀ measured perpendicular to outer surface 154 from inner surface 156 to outer surface 158. Total thickness T₁₅₀ is less than 0.030 in. As previously described, thicknesses T₁₅₁, T₁₅₂ are substantially uniform, and thus, total thickness T₁₅₀ is also substantially constant or uniform moving laterally along coating 150. The first layer 151 of protective coating 150 has a first hardness that is greater than 1000 HV and the second layer 152 of coating 150 has a second hardness of about 850 HV.

Although coating 150 is shown and described as including two layers 151, 152, in other embodiments, the multilayered protective coating (e.g., coating 150) includes more than two layers. However, in such embodiments, each layer preferably has a thickness less than 0.005 in. (measured perpendicular to the outer surface of the underlying base metal or material), and the coating preferably has a total thickness less than about 0.030 in. (measured perpendicular to the outer surface of the underlying base metal or material).

Referring now to FIG. 3, an embodiment of a multilayered coating 170 for protecting an underlying base metal or material 173 is shown. For example, coating 170 can be used in place of coating 150 previously described to enhance the wear and corrosion resistance of a downhole tool. In this embodiment, coating 170 includes a first layer includes a first layer 171 deposited directly onto the outer surface 174 of base material 173 and a second layer 172 deposited directly onto the first layer 171. Thus, first layer 171 is positioned between base material 173 and second layer 172. First layer 171 comprises a first material 171 a and has a first thickness T₁₇₁ measured perpendicular to outer surface 174, and second layer 172 comprises a second material 172 a and has a second thickness T₁₇₂ measured perpendicular to outer surface 174. In this embodiment, first material 171 a comprises Ni—P deposited onto surface 174 via a electroless process, and second material 172 a comprises chromium deposited by an electrolytic process directly onto first layer 171 at a discrete current density (A/in²).

In this embodiment, thickness T₁₇₁ of layer 171 is substantially constant and uniform moving laterally along coating 170, and thickness T₁₇₂ of layer 172 is substantially constant and uniform moving laterally along coating 170. Thickness T₁₇₁, T₁₇₂ of each layer 171, 172, respectively, is preferably less than 0.0050 in. More specifically, thickness T₁₇₁ is preferably between about 0.00020 in. and 0.0030 in. and thickness T₁₇₂ is preferably between about 0.00050 in. and about 0.0050 in.

Coating 170 has an inner surface 176 engaging outer surface 174 of base material 173, an outer surface 178 distal to the base material 173, and a total thickness T₁₇₀ measured perpendicular to outer surface 174 from inner surface 176 to outer surface 178. Total thickness T₁₇₀ is less than 0.030 in. As previously described, thicknesses T₁₇₁, T₁₇₂ are substantially uniform, and thus, total thickness T₁₇₀ is also substantially constant or uniform moving laterally along coating 170. In addition, second layer 172 has a micro-crack density between 400 and 850 micro-cracks per inch, and more specifically between 400 and 650 micro-cracks per inch.

Although coating 170 is shown and described as including two layers 171, 172, in other embodiments, the multilayered protective coating (e.g., coating 170) includes more than two layers. However, in such embodiments, each layer preferably has a thickness less than 0.005 in. (measured perpendicular to the outer surface of the underlying base metal or material), the coating preferably has a total thickness less than about 0.030 in. (measured perpendicular to the outer surface of the underlying base metal or material), and the layers of Ni—P and chromium are preferably arranged in an alternating fashion.

Embodiments described herein also include methods for making or forming a wear and corrosion resistant coating on an outer surface of a base metal or material. In one embodiment, the method comprises: (a) depositing a first layer of a first material onto the surface of the base material; and (b) depositing a second layer of a second material onto the first layer after (a); wherein the first material or the second material comprises chromium and is deposited at a current density of less than about 4.0 A/in². In some embodiments, the first material (e.g., material 151 a) and the second material (e.g., material 152 a) each comprise chromium; wherein (a) comprises depositing the first layer of the first material at a first current density; and wherein (b) comprises depositing the second layer of the second material at a second current density that is different than the first current density. In another embodiment, one of the second current density and the first current density is about 3.5 A/in² and the other of the first current density and the second current density is about 1.0 A/in². In further embodiments, the first current density may be about 3.0 A/in², 2.5 A/in², 2.0 A/in², 1.5 A/in², 1.0 3 A/in², and 0.5 A/in². In still further embodiments, the second current density may be about 3.0 A/in², 2.5 A/in², 2.0 A/in²; 1.5 A/in², 1.0 A/in², and 0.5 A/in².

In another embodiment, depositing the first layer of the first material may be by pulse current; and depositing the second layer of the second material may also by pulse current.

In some embodiments of a method of coating a base surface, the second layer formed from a second material is deposited until the layer has a thickness of about 0.0002 in. to 0.0003 in. measured perpendicular to the surface of the base material. Similarly, in some embodiments, the first layer of the first material is deposited until the first layer has a thickness of about 0.00020 in. to 0.0003 in. also measured perpendicular to the surface of the base material. Embodiments of such a method wherein the first material consists of chromium and the second material consists of chromium are illustrated in FIGS. 2 and 4(B).

Referring now to FIG. 5, an embodiment of a method 200 for making coating 150 as previously described is schematically shown. Beginning in block 201 of method 200, first material 151 a comprising chromium is deposited onto base material 153 by a pulsed or alternative current electroplating to form layer 151. The chromium of first material 151 a is applied to the base material 153 at a first current density, in for example a Heef® 25 bath, where the length of time that the current is applied and the concentration of chromium ions determines the thickness of the layer. Next in block 202, a second material 152 a comprising chromium is deposited onto first layer 151 by a pulsed or alternative current electroplating to form layer 152. The chromium of second material 152 a is applied to first layer 151 at a second current density that is different than the first current density at which first material 151 a is applied, however, each of the current densities is preferably less than 4.0 A/in².

As shown in block 203, blocks 201 and 202 may be repeated as necessary to produce a coating on surface 154 comprising any desired number of discrete layers (e.g., layers 151, 152) of chromium, as well as any desired total thickness (e.g., total thickness T₁₅₀) that is preferably less than 0.03 in.

Referring now to FIG. 6, an embodiment of a method 300 for making protective coating 170 as previously described is schematically shown. Beginning in block 301 of method 300, a first material 171 a comprising Ni—P is deposited onto base material 173 by an electroless process. In block 303, the Ni—P (first layer 171) is heated at about 375° F. for about 1.5 hr., and further heated at 500° F. for about 1 hr, wherein the first layer 171 is heated for a total of 2.5 hrs. In this embodiment, the resultant layer 171 is about 0.0010 to about 0.0020 inches thick, has an increased hardness of about 50 Rc. Next, in block 303, Second material 172 a comprising chromium is deposited on first layer 171 by a pulsed or alternative current electroplating to form second layer 172. The current density at which second material 172 a is applied is preferably less than 4.0 A/in².

As shown in block 304, blocks 301, 302, 303 may be repeated as necessary to produce a coating on surface 174 comprising any desired number of discrete layers (e.g., layers 171, 172) of Ni—P and chromium, and any desired total thickness (e.g., total thickness T₁₇₀) that is preferably less than 0.03 in.

Although coating 150 was shown and described in connection with bit body 110, in general, embodiments of coatings described herein (e.g., coatings 150, 170) can be applied to the surface of any downhole tool such as but not limited to mandrels, mud motor rotors, and agitator rotors. In one embodiment a down-hole tool comprises a body made of a base material and a protective coating is mounted to an outer surface of the base material. The protective coating comprises a first layer deposited directly onto an outer surface of the base material; and a second layer deposited directly onto the first layer, wherein the first layer is positioned between the base material and the second layer; wherein the first layer comprises chromium having a first micro-crack density and the second layer comprises chromium having a second micro-crack density that is less than the first micro-crack density.

In another embodiment, the first micro-crack density is greater than 2500 micro-cracks per inch and the second micro-crack density is between 1000 and 1500 micro-cracks per inch, in a further embodiment, the first micro-crack density is greater than 1500 micro-cracks per inch and the second micro-crack density is between 500 and 850 micro-cracks per inch, and in a preferred embodiment the first micro-crack density is greater than 1000 micro-cracks per inch and the second micro-crack density is between 400 and 650 micro-cracks per inch.

In some embodiments of the tool described herein, the first layer has a first thickness measured perpendicular to the outer surface of the base material, and the second layer has a second thickness measured perpendicular to the outer surface of the base material; wherein the first thickness of the first layer is substantially uniform and the second thickness of the second layer is substantially uniform. In another embodiment, the second layer has a thickness between about 0.00005 in. and about 0.020 in. measured perpendicular to the outer surface of the base material, and in a preferred embodiment second layer has a thickness between about 0.00050 in. and about 0.0020 in. measured perpendicular to the outer surface of the base material.

In a further embodiment of the tool described herein, the coating has an inner surface engaging the outer surface of the base material and an outer surface distal the base material, and wherein the coating has a total thickness less than 0.050 in. measured perpendicularly from the outer surface of the base material to the outer surface of the coating; in a further still embodiment, the coating has an inner surface engaging the outer surface of the base material and an outer surface distal the base material, and wherein the coating has a total thickness less than 0.030 in. measured perpendicularly from the outer surface of the base material to the outer surface of the coating. In another embodiment of the tool the first layer has a first hardness greater than 1100 HV and the second layer has a second hardness of about 500 HV; in a further embodiment the first layer has a first hardness greater than 1000 HV and the second layer has a second hardness of about 650 HV; and in a preferred embodiment the first layer has a first hardness greater than 1000 HV and the second layer has a second hardness of about 850 HV.

EXAMPLES Example 1

Production of a wear and corrosion resistant coating on the surface of a downhole tool (e.g., surface 150) in accordance with principles described herein.

In one embodiment herein described, a 6.75 inch agitator was hard chrome plated in a Heef® 25 bath with alternative current densities of 2.0 and 4.0 A/in². The coated agitator was subjected to field runs in conditions: water based mud of 8.4 to 10.25 ppg with pH of 7.8 to 11 at 170-176° F. The mud contained 0.2 to 13.9% solid, 0.25% sand, and 154,000 to 160,000 mg/l of chlorides content. The conventional hard chrome plated agitator rotor is not recommended to run in chloride concentrations of >100,000 mg/l. The total run time of alternated current hard chrome plated agitator rotor was 456 hours of 10 field test runs

Example 2

Production of a wear and corrosion resistant coating on a base material (e.g., coating 150) in accordance with principles described herein.

Alternative current electrolytic plating as described herein was used to create a multilayer Chromium coating (FIG. 5B), and the microstructure of the coating was compared to a single layer hard chrome plating of the prior art (FIG. 5A). The coating comprising of multiple layers of Cr plating can be visualized in the SEM image of FIG. 5(B). Micro-cracks in the cross-sectional polished surface of each of the coatings were enhanced by etching with Mable reagent.

The Cr plating of the prior art (FIG. 5A) was deposited with current density of 2 A/in². The micro-cracks in the conventional hard Cr plating were large, and in the deposition (grow) direction, perpendicular to the surface of the base material.

In the embodiment of the protective coating described herein, the multilayer Cr plating (the darker layer or first chrome layer) was deposited at 3.5 A/in²; while the lighter layer or second chrome layer) was deposited at 1.0 A/in². The low current density layer (second layer) has a denser microstructure, as evidenced in the FIG. 5B; this is due to the fact that the lower the current density, the slower the metal ion deposit time, the dense the product, and the greater the number of micro-cracks per inch. Further, it can be seen in the first layer that was plated at a current density of 3.5 A/in², that some cracks seemed to orientate parallel to the plating surface. As is described herein, the concentration of metal ions, length of deposition, and current density can all be varied to create a coating that satisfies the required wear and corrosion resistance.

SUMMARY OF FEATURES AND ADVANTAGES

Embodiments of the invention described herein provide for various coatings for application to downhole tools, wherein the coating provides enhanced wear and corrosion resistant coatings. Methods for producing such coatings are also provided.

Various embodiments of current density are employed to produce a plurality of layers that in some embodiments comprise chrome, each can be of a different thickness and/or micro-crack density. Micro-cracks are thus specific to one layer, rather than to the entire coating (as seen in prior art hard chrome coatings that comprise one layer), and function to reduce the degree to which corrosive fluids (for example from drilling environments) can penetrate the entire thickness of the coating and access the base material of the underlying tool, causing corrosive and wear damage.

Hence the presence of multiple layers in the coating reduce the degree to which the coating is susceptible to wear and erosion. Further such a microstructure comprising the described micro-crack densities are desirable, because the deposited surfaces also have lower stress, higher lubricity, and enhanced wearability.

In one embodiment of the method of making a coating for protecting a base material, the base material is a matrix drill body, and in a further embodiment, the coating may be applied to any surface in need of improved corrosive resistance, and or wear resistance, such as but not limited to downhole drilling equipment or tools.

Therefore it is believed that the protective coatings made by the methods described herein and exemplified in examples described herein, will impart to a surface and such downhole tools as drill bit bodies and wear surfaces to which said materials are applied, improved wear resistance and corrosive resistance as compared to some conventional protective coatings, downhole tools, bit bodies and wear surfaces.

While preferred embodiments have been shown and described, modifications thereof can be made by one skilled in the art without departing from the scope or teachings herein. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the systems, apparatus, and processes described herein are possible and are within the scope of the invention. For example, the relative dimensions of various parts, the materials from which the various parts are made, and other parameters can be varied. Accordingly, the scope of protection is not limited to the embodiments described herein, but is only limited by the claims that follow, the scope of which shall include all equivalents of the subject matter of the claims. Unless expressly stated otherwise, the steps in a method claim may be performed in any order. The recitation of identifiers such as (a), (b), (c) or (1), (2), (3) before steps in a method claim are not intended to and do not specify a particular order to the steps, but rather are used to simplify subsequent reference to such steps. 

What is claimed is:
 1. A coating for protecting a base material from wear and corrosion, the coating comprising: a first layer deposited directly onto an outer surface of the base material; and a second layer deposited directly onto the first layer, wherein the first layer is positioned between the base material and the second layer; wherein the first layer comprises chromium having a first micro-crack density and the second layer comprises chromium having a second micro-crack density that is less than the first micro-crack density.
 2. The coating of claim 1, wherein the first micro-crack density is greater than 1000 micro-cracks per inch and the second micro-crack density is between 400 and 650 micro-cracks per inch.
 3. The coating of claim 2, wherein the first layer has a first thickness measured perpendicular to the outer surface of the base material and the second layer has a second thickness measured perpendicular to the outer surface of the base material; wherein the first thickness of the first layer is substantially uniform and the second thickness of the second layer is substantially uniform.
 4. The coating of claim 2, wherein the second layer has a thickness less than 0.0050 in. measured perpendicular to the outer surface of the base material.
 5. The coating of claim 2, wherein the second layer has a thickness between about 0.00050 in. and about 0.0020 in. measured perpendicular to the outer surface of the base material.
 6. The coating of claim 2, wherein the first layer has a first hardness greater than 1000 HV and the second layer has a second hardness of about 850 HV.
 7. The coating of claim 6, wherein the coating has an inner surface engaging the outer surface of the base material and an outer surface distal to the base material, and wherein the coating has a total thickness less than 0.030 in. measured perpendicularly from the outer surface of the base material to the outer surface of the coating.
 8. The coating of claim 7, wherein the first layer has a thickness between about 0.00020 in. and 0.0030 in. measured perpendicular to the outer surface of the base material.
 9. A method for forming a wear and corrosion resistant coating on a surface of a base material, the method comprising: (a) depositing a first layer of a first material onto the surface of the base material; and (b) depositing a second layer of a second material onto the first layer after (a); wherein the first material or the second material comprises chromium and is deposited at a current density of less than about 4.0 A/in2.
 10. The method of claim 9, wherein the first material and the second material each comprise chromium; wherein (a) comprises depositing the first layer of the first material at a first current density; and wherein (b) comprises depositing the second layer of the second material at a second current density that is different than the first current density.
 11. The method of claim 10, wherein one of the second current density and the first current density is about 3.5 A/in2 and the other of the first current density and the second current density is about 1.0 A/in2.
 12. The method of claim 10, wherein (a) comprises depositing the first layer of the first material by pulse current; and wherein (b) comprises depositing the second layer of the second material by pulse current.
 13. The method of claim 10, wherein (b) comprises depositing the second layer of the second material until the second layer has a thickness of about 0.0002 in. to 0.0030 in. measured perpendicular to the surface of the base material.
 14. The method of claim 13, wherein (a) comprises depositing the first layer of the first material until the first layer has a thickness of about 0.00020 in. to 0.0030 in. measured perpendicular to the surface of the base material.
 15. The method of claim 9, wherein the first material consists of chromium and the second material consists of chromium.
 16. The method of claim 9, wherein the first material comprises Ni—P and the second material comprises chromium.
 17. The method of claim 16, wherein (a) comprises depositing the first layer of the first material by an electroless process.
 18. The method of claim 17, wherein (b) comprises depositing the second layer of the second material until the second layer has a thickness of about 0.0002 in. to 0.0030 in. measured perpendicular to the surface of the base material.
 19. The method of claim 16, further comprising (c) heating the first layer after (a) and before (b).
 20. The method of claim 19, wherein (c) comprises: (c1) heating the first layer at about 375° F. for about 1.5 hr.; and (C2) heating the first layer at about 500° F. for about 1 hr after (c1).
 21. The method of claim 20, wherein (c) comprises increasing the hardness of the first layer to about 50 Rc.
 22. A down-hole tool comprising: a body made of a base material; a protective coating mounted to an outer surface of the base material, wherein the protective coating comprises: a first layer deposited directly onto an outer surface of the base material; and a second layer deposited directly onto the first layer, wherein the first layer is positioned between the base material and the second layer; wherein the first layer comprises chromium having a first micro-crack density and the second layer comprises chromium having a second micro-crack density that is less than the first micro-crack density.
 23. The tool of claim 22, wherein the first micro-crack density is greater than 1000 micro-cracks per inch and the second micro-crack density is between 400 and 650 micro-cracks per inch.
 24. The tool of claim 23, wherein the first layer has a first thickness measured perpendicular to the outer surface of the base material and the second layer has a second thickness measured perpendicular to the outer surface of the base material; wherein the first thickness of the first layer is substantially uniform and the second thickness of the second layer is substantially uniform.
 25. The tool of claim 23, wherein the second layer has a thickness between about 0.00050 in. and about 0.0020 in. measured perpendicular to the outer surface of the base material.
 26. The tool of claim of claim 25, wherein the coating has an inner surface engaging the outer surface of the base material and an outer surface distal the base material, and wherein the coating has a total thickness less than 0.030 in. measured perpendicularly from the outer surface of the base material to the outer surface of the coating.
 27. The tool of claim 23, wherein the first layer has a first hardness greater than 1000 HV and the second layer has a second hardness of about 850 HV.30.
 28. A coating for protecting a base material from wear and corrosion, the coating comprising: a first layer deposited directly onto an outer surface of the base material; and a second layer deposited directly onto the first layer, wherein the first layer is positioned between the base material and the second layer; wherein the first layer comprises Ni—P and the second layer comprises chromium having a micro-crack density between 400 and 650 micro-cracks per inch.
 29. The coating of claim 28, wherein the second layer consists of chromium.
 30. The coating of claim 29, wherein the first layer has a first thickness measured perpendicular to the outer surface of the base material and the second layer has a second thickness measured perpendicular to the outer surface of the base material; wherein the first thickness of the first layer is substantially uniform and the second thickness of the second layer is substantially uniform.
 31. The coating of claim 28, wherein the first layer has a first thickness measured perpendicular to the outer surface of the base material and the second layer has a second thickness measured perpendicular to the outer surface of the base material; wherein the first thickness is between 0.0005 in. and 0.002 in. and the second thickness is between 0.0002 in. and 0.0030 in.
 32. The coating of claim of claim 28, wherein the coating has an inner surface engaging the outer surface of the base material and an outer surface distal the base material, and wherein the coating has a total thickness less than 0.030 in. measured perpendicularly from the outer surface of the base material to the outer surface of the coating. 